A variable which, when set equal to 1, recognizes the "If-Modified-Since" header coming from browsers and allows browsers to use locally cached pages. Disabled by default to help the administrator customize its page without needing permanent reloading.

This activates an internal stopwatch that shows how long it takes to render a page. (If you have a wiki that composes a HTML page from multiple pages, such as a normal layout with a sidebar, you'll get separate timings for each subpage and for the total page.)

The timings can be displayed by adding <!--function:StopWatchHTML 1--> in the wiki template.

Is it possible for someone with admin priviledges to always have access to debugging tools, without letting everyone else access them?

You can easily & automatically allow debugging for anyone with admin priviledges (meanwhile leaving it off for everyone else) by including this line in config.php - just be sure that 1) $EnableDiag is either null or set to 0, and 2) to include it near the end of config.php, AFTER declaring your passwords, and after any AuthUser or other priviledge settings: